Biasing system for magnetic recorders



July 22, 1952 L, CQNNELL 2,604,546

BIASING SYSTEM FOR MAGNETIC RECORDERS Filed Aug. 23, 1948 3 Sheets-Sheet 1 T0 nsconnms HEAD INPUT SIGNAL TO B RECORDED INVENTOR.

LAWRENCE H. GONNELL memo o v i 7" ATTORNEYS July 22, 1952 CQNNELL 2,604,546

. BIASING SYSTEM FOR MAGNETIC RECORDERS Filed Aug. 23, 1948 3 Sheets-Sheet 2 MM M ww W

a w 1w J L Ju M W1 Q WW)! FIG.2. f I w INVEN TOR.

LAWRENCE H. CONN ELL fill y 22, 1952 L. H. CONNELL 2,604,546

BIASING SYSTEM FOR MAGNETIC RECORDERS Filed Aug. 25, 1948 a Sheets-Sheet 3 FIG 4 INVENTOR. LAWREN CE H. CONNELL M ATTORNEYS Patented July 22, 1952 UNITED STATES PATENT Omar],

BIASING SYSTEM FOR MAGNETIC RECORDERS Lawrence H. Connell, Detroit, Mich.

Application August 23, 1948, Serial No. 45,711

20 Claims.

The present invention relates to method and apparatus for magnetic recording, and has for its primary object to so improve the quality of such recordings that they may be satisfactory in fields where fidelity is a requirement such as the recording and reproduction of music.

It is a further object of the present invention to so reinforce or augment the effect of a sound modulated controlling force that the strength of the magnetic field controlled thereby through which a record element is advanced is, while changing polarity in accordance with the sign or polarity of a sound pattern,- at all times of a magnitude sufficient to efficiently impart a residual magnetization to the record element capable of reproduction.

More specifically it is an object of the present invention to provide apparatus and method for sound recording which operates from an audio pattern in which sound waves of reversing sign or polarity and of a magnitude which is variable from zero upwardly in both directions is used to control a magnetic field in such a way that the magnetic field reverses polarity substantially synchronously with reversals in sign or polarity of the audio pattern, andin which the magnetizing effect of the magnetic field varies directly with variations in the magnitude of the audio pattern.

It is a feature of the present invention to control a magnetizing current by two voltages, one of which is variable in magnitude and reverses sign or polarity in accordance with an audio pattern, the other of which reverses sign or polarity substantially simultaneously with the first, and which has the effect of insuring that the magnetizing current, except during the instant of reversel, is-never below a magnitude sufiicient to produce efficient recording.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a BH diagram for the initial magnetization of a permanent magnet material suitable for use as a record element;

Figure 2 is a diagram illustrating the additive relationship of currents controlled by an audible sound pattern to be recorded and a reinforcing current of substantially uniform magnitude to the same instantaneous polarity as the sound modulated current;

Figure 3 is a block diagram disclosing a circuit for carrying out the present invention;

Figure 4 is a more detailed circuit diagram of the circuit employedy Figure 5 is a chart showing input during recording plotted against output during playback; and

Figure 6 is a B -H diagram for the initial magnetization of a different permanent magnet material.

Referring first to Figure 1, there is illustrated a BH curve typical of permanent magnet materials suitable for record elements. This chart represents magnetization (B) for different values of magnetizing force (H), assuming the material to be unmagnetized prior to subjection to the magnetizing force, a condition which is substantially achieved in the present invention. It will be observed that from the origin to the symmetrically located points a, a, the magnetizing effect is negligible for magnetizing forces represented by'h, 11'. Beyond the points a, a, the curve is sensibly straight, and it will be noted that values it, h are selected such that the prolongations of the straight intermediate portions of the curve intercept the H axis substantially at the selected values. For all values of magnetizing force above h, h and to substantial saturation of the material, the response is 'linear,.. and magnetization of the record element. is directly proportioned to the magnetizing force. This permits recording and reproduction of sounds of variable intensity at their low relative intensity or magnitude, so long as the magnetizing force attributable thereto exceeds the values 71., it. However, if the magnetizing force varies directly with the magnitude of the audio signal, low intensity sounds will produce magnetizing forces falling betweenthe points a, a on the curve, and these magnetizing forces are incapable of producing reproducible recordings.

This fundamental difficulty has been recognized in the past, and efforts made to; overcome it. For example, it has previously been proposed to add a supersonic component to the magnetizing current supplied'to the electromagnet used in recording. This has the effect of eliminating or reducing the hysteresis effect of the record element, and'permits recording to include low intensity sounds as well as high intensity sounds. However, when employed with conventional recordingheads, iteither fails to record or acts to erase or partially erase very high audio frequencies, and further it results in amplitude and frequency modulation of medium high audio frequencies. v

A second method intended to overcome "the inherent difliculties in effecting recording of sound at-true relative intensities involved the-use of a direct current bias,.or partial magnetization 3 of the record element prior to recording. This of course limits the portion of the BH curve used and fails to take full advantage of the recording potentials of the record element.

According to the present invention, the audio signal is used to produce a first variable controlling force, such as a voltage, which reverses sign or polarity substantially simultaneously with reversals of the signal, and which varies in magnitude directly with the audio signal. The audio signal or the first variable controlling force is used to produce a second controlling force of like character (such as voltage) which'however, while it changes sign or polarity substantially simultaneously with the first, is of a predetermined and preferably substantially constant magnitude.

These two controlling forces are used to control a magnetizing force, as for example ,the current in the windings of an electromagnet, so that the current is proportional to the sum of the two controllin for es.

Therefore, if the magnetizing force attributable to the second of said controlling forces is equivalent to the ma eti n forces e 7 low intensity sounds will b recorded at their true relative intensity. The valu of the forces h. h. are selected such that any e o attributable to these forces aIQne is below the noise level.

Referring 110W to Figure 2 there is shown a curve plotted alon the axis A which the pattern of an audio modulated controlling force, such as current, voltage. bight, or other force. It will be observed that this ,force varies considerably in intensity and also changes its sign or polarity. At the point of the curve designated m there is shown a force of moderate magnitude which includes minor peaks of relatively high frequency superimposed upon a peak of relatively low frequency. At 11. there is represented a relatively large peak of force. If the force represented in the foregoing is used to produce a corresponding' current applied to the windings of an electromagnetfth magnetic field established by low valuesof this current will be insufiicient to effect permanent magnetization of a record element. I

"According to the present invention the force illustrated "at A is reinforced or augmented by the force represented at B. In this case it will be observedthat the force ,13 is ofsubstantially uniform magnitude and is always of the same sign or polarity as the force plotted at A.

' The invention contemplates the method of and means for producing" a magnetizing current equivalent to the resultantforce which would be obtained by 'add'ingthe forces plotted at A and'B, 'as'shown at'C. It will be observed that the curve at 0 results from the'simple addition of values represented by the curves A and B; It will'further be observed that 'all values of the audio modulated current are thus increased to a relatively high valu sufiic'ient to efieot permanent" recording without the introduction of any appreciable frequency or relative amplitude distortiori.

In order tocarry out the foregoing, the apparatus diagrammatically iuustratec in Figure' 3 is employed. In this figure'anfinc'oming signal of audibledrequency' is applied to'the grid" of an amplifier tube l which may be a high gain triode, such as a 6535. The same indicating sig nalisapplied directly to a mixing circuit, such asto a gridof a'inixing 'tubels'uch as a GUI. The signal will be of such magnitude that both. tubes I 4 operate as linear amplifiers. The output of the amplifier is fed through a limiting resistor 2 to a biased dual diode 3, such as a 6H6 tube. The bias for the dual diode 3 may be furnished by constant voltage dry cells. The output of the tube 3 is therefore an amplified version of the original signal with its phase shifted degrees and with its peak values :limited' to the voltage of the dry cellf :This output is'ifedto another high gain amplifier 4, such as a SSFS', so that it will handle the voltage of one dry cell applied to its grid without distortion. Again, the output of the amplifier 4 is limited and clipped by another dual diode 6 identical with the dual diode 3, a limiting resistor 4 being interposed. Bias for the dual diode 6 is furnished by the same dry cells which furnish the bias for the dual diode 3. The output is accordingly a substantially square wave and a second phase shift of 180 degrees taken place .so that the sign or polarity of the output'is the same as that of the original signal. Theldesired portion of the resulting squar wave is selected by .a voltage divider 1 and is thenfed into the other control grid of the ,mixingltube .9. Accordingly the two control grids of tube 9 are independently operated; one by'a voltage of variable magnitude and reversing polarity corresponding to the audio pattern of the signal to be recorded; and the other by a voltage which is of substantially constant magnitude andwhich reverses its polarity substantially simultaneous y W th e first voltage soras to be at all times of the same polarity therewith. The output of the mixing tube 9 is fed to an amplifier such as a 6L6 diagrammatically represented at l0, theoutput of which is fed through a blocking-condenser to the windingsof the electromag-net of the recording head. t i

Satisfactory results are obtained if resistors 2 and 5 are .1 megohm and the grid leak l2 and voltage divider Lare .5 megohm. Except for precautions noted hereafter, other electrical values in the system are conventional and will b readily apparent to those skilled the art. v

The amplifierifl is preferably a pentode which is chosen as it has a very high plate resistance and in combination with the recording head it will producea current sensibly proportional to the grid voltage applied thereto. It is desirable that the cathodes and screens-have very low impedance, i. e. very large capacity to ground. Further the plate circuitsshould be thoroughly decoupled from each other. The recording head windings are highly inductive. At the time the polarit'y'of the signal changes, the pentode gencrates a high voltage across their windings in an effort to make the current follow the voltage. There is thus a tendency to oscillate. This oscillation isbetween the inductance of the head and stray capacity to ground. Careful reduction of this stray greg iacity to ground eliminates the oscillation or makes it of such high frequency that it is undetectable on an oscilloscope.

While the foregoing is believed sufiicient to constitute an adequate disclosure of the circuits employed, Figure l is presented to give a detailed disclosure of the various elements of the circuit, the diagram being self-explanatory in the light of the foregoing. It will be apparent to one skilled in the art that currents through suitable networks could be added to obtain th desired field, rather than controlling this field by impressing the voltages referred to above on the gridsof a mixing'tube. iIt willst'illicontinue im portant to maintain phase relationship.

. In setting up'the system for optimum results; vacuum-tube volt meters are connected to each grid of the mixing tube 9. A signal of fixed frequency from an audio oscillator isfe'd in. The volt meter indicating the output for the voltage divider 1 will stay. almost constant for any given setting of the voltage divider even though the input may. be varied over a wide range. A setting of the voltage divider 1 is chosen and a record. is made with various input voltages. This record is played backuand output voltages are measured. These values ar plotted as indicated in Figure 5 in which are illustrated three curves representing output during playback against in-'- put during recording. It is desirable to obtain a curve such that its relatively straight intermediate portion if projected would pass through the zero-zero point of the chart. If the initial testing results in a curve such as shown at L, which represents a condition resulting from low amplification, the output of thevvoltage divider is increased. If the test develops a curve such as shown at K, which is the condition which would result from too high amplification, the output of the voltage divider, is reduced. The optimum results are obtained when the test produces a curve indicated 'atP," which, it will be observed, includes a relatively straight intermediate portion whose projection passes substantiallyv through the'zero-z'ero point 001' the chart.

Recordings made in accordance with the present invention will operate torecord with surprisingly high fidelity to very high audible frequencies. The results obtained are in part dependent uponthe use of a recording head characterized by a very small and effectively constant gap width. With proper selection of gap'width and design of recording head in a manner set forth in considerable detail in my prior copending. application Serial No. 43,288, filed August 9, 1948, it is possible to obtain high fidelity recording throughout the entire audible range. v

The foregoing description applies to a relatively simple case in which a constant magnetizing force in effect breaks the curve for the initial magnetization of permanent magnetic material into two linear parts. This has proved entirely satisfactory in practicebut of course it depends upon the fact that the intermediate portions of the B- -H curve intermediate saturation inopposite directions and the 'zero point are sensibly straight lines. If a magnetic material is employed in which these intermediate "portions of the curve are not sensibly straight lines, a further correcting approximation may be made by dividing these curves-into three parts as indicatedin Figure 6, the parts being O- -X, XY and. Y''-Z. The. correction for the-effect of the portion O-A would be accomplishedas .already described except that the constantmagnetizing force, instead of goingdirectly to the grid of the mixing tube, would go to a resistance network. A biased amplifier is ,providedwhich will begin amplification for a voltage corresponding to the point X and be limited at a voltage correspond! ing to the point Y. Precautions will of'course be taken to keep the phase angle correct. The output would goto. another arm of thejresistance network which would combine the signals in the correct portions and apply them to the grid of the mixing tube. v

Furthermore thesame general theory may be employed to correct the portion of the curve beyond the point 2 and complete saturation. In order to obtain the effect of correct proportional recording, a low time constant biased expander would be installed 'in the incoming signal lead. The bias would be such that the expander would start operation at a point corresponding to the point Z, and have such an expansion factor as to make the curve beyond Z'as nearly as possible a continuation of the curve YZ. The extension of the curve is indicated by dotted linesin the figure. v 1

Throughout the foregoing specification and in the claims, reference is made to the magnetic re cording of sound. It will be understood that the invention is applicable to the recording of other signals, information or data whose frequency is within the range of the equipment.

The drawings and the foregoing specification constitute a description of the improved method and apparatus for magnetic recording in such full, clear, concise and' exact terms as to enable any person skilled in the art to practice th invention, the scope of which is indicated by the appended claims. I p 7 What I claim as my invention is:

1. In a recorder for magnetically recording sound which comprises a recording head provided with an electromagnet having spaced poles adapted to be engaged by a continuously advanced record element formed of permanent magnet material, and windings for said electromagnet; a circuit for connection to the Windings comprising means for supplying a first voltage of variable magnitude and reversing polarity corresponding to a sound pattern to be recorded, other means for supplying a second voltage of reversing polarity substantially in phase with the first voltage, and means responsive to said voltages for establishing avcurrent in the windings which varies in accordance with the additive values of, said voltages.

y 2. In a recorder for magnetically recording sound which comprises a recording head provided with an electromagnet having spaced poles adapted to be engaged bya continuously advanced record element formed of permanent magnet material, and windings for said electromagnet; a circuit for connection tothe windings comprising means for supplying a first voltage of variable magnitude and reversing polarity corresponding to a sound pattern to be recorded, other means for supplying a second voltage of substantially constant magnitude and reversing polarity substantially in phase with the first voltage, and means responsive to said voltages for establishing a current in the windings which varies in accordance with the additive values of said voltages. V

3. A recorder for magnetically recording sound which comprises arecording head provided with an electromagnet having spaced poles adapted to be engaged by a continuously advanced record element formed of permanent magnet material, and windings for said electromagnet; a

circuit for said windings comprisinga mixing tube, an output circuit connecting said tube to said windings, said tube having a pair of grids means for impressing on one of said grids a voltage which varies in magnitude and polarity in accordance with a sound pattern tobe recorded, and means for impressing on the other of. said grids a voltage which is of substantially constant magnitude and the same instantaneous polarity as thevoltage' impressed on the said one grid."

aeoatee 4. Apparatus tor making magnetiasound reomin s wh ch comprises m ans for esta l shin a magnetic field, means for varying the magnitude and polarit of the field comprising first control means operable to effect variations in magnitude and polarity of the field in accordance with .a soundpattern .to be recorded, and second control means operable to effect variations in the field which are-of the same instantaneous 'polarityas the variations efiected by said first control means.

5. Apparatus as defined in claim 4 in which said ,Second control means is operable to effect field variations of uniform magnitude.

6. Apparatus for making magnetic sound recordings which comprises first means for establishing a sound modulated magnetic field controlling force which varies in magnitude'and polarity, and a second means for establishing a .second magnetic field controlling force which is of the same instantaneous polarity as the force established by said first means. 1

7. Apparatus as defined in claim 6 in which said second means establishes a controlling force of substantially constant magnitude.

8. Apparatus for recording sound magnetically on a record element formed of permanent magnetmaterial which comprises means ,for establishing a magnetic field means foradvancing said record element continuously through the field, and means for varying the magnetizing effect of the fieldcomprising first control means operable to establish a sound modulated field controlling force, and second control means operable to establish a field controlling force which is of the same instantaneous polarity as the force established by said .firstmeans. I

9. Apparatus as defined in clainrS in which said second control means establishes a force of constant magnitude.

10. Apparatus as defined in claim 9 in which the magnitude of the force established by said second control means is sufiicient to causevariati'ons in the magnetic field substantially equal to the coercive force of a record element on which the record isto bemade. 1

11. Apparatus for; recording sound magneticallyon a recordelement oi. permanent magnet material comprising an. electro-magnet having spaced poles and energizing windings, means for advancing a record element continuously through the field adjacent said poles, and means for es.- tablishing a .magnetizing current in said windings which is equivalent to.the resu1t of the addition of a sound modulated current varying in-magnitude and polarity with anadditive current of the same instantaneous polarity as the sound modulated current. v V,

12. Apparatus for recording soundmagnetically on -a record element of permanent magnet material comprising an .electroe-magnet having spaced poles and energizing windings, means for advancing a record element continuously through the field adjacent said poles, and means for establishing a magnetizing current in said windings which is equivalent to the result of the addition of a sound modulated currentvarying in magnitude and polarity with an additive current of constant magnitude and thesame instantaneous polarity'as the sound modulated current.

13. Apparatus for. recording sound magnetically on a recordv element of permanent magnet mate al ompris a electro-ma net havin spaced poles and energizing windings, means for advancing a record element continuouslythrough the field adjacent said poles, and means for establishing amagnetizing current in said windings which is equivalent to the result of the addition of a symmetrically sound modulated current varying in magnitude and polarity with anadditive current of constant magnitude and the same instantaneous polarity as the sound modulated current.

14. Apparatus for, magnetic sound recording comprising a control circuit for a magnet including a mixin vacuum'tube having two grids, means for applying a sound modulated voltage varying in magnitude and polarity to one of said grids, and means for applying to the other grid a separate constant voltage of the same instantaneous polarity as the voltage on the first grid.

15. Apparatus for magnetic sound recording comprising a control circuit for a magnet including a mixing circuit having a plurality of branches, means for applying to one branch of said mixing circuit a sound modulated voltage varying in magnitude and polarity, and means for ap lying to another branch of said mixing circuit a separate voltage of the same instantaneous polarity as the voltage applied to said first branch.

16. Apparatus for magnetic sound recording comprising a control circuitf'for a magnet including a mixingcircuit having a plurality of branches, means for applying to one branch of said mixing circuit asound modulated voltage varying in magnitude and polarity, and means for applying to another branch of said mixing circuit a separate. constant voltage of the same instantaneous polarity as the voltage applied to said first branch.

17. Apparatus for .magnetic sound recording comprising a control circuit for a magnet includinga mixing circuit having a plurality of branches, means for applying to one branch of said mixing circuit a sound modulated voltage varying symmetrically in magnitude and polarity, and means for applying to another branch of said mixing circuit a separate constant voltage of the same instantaneous polarity as the voltage applied to said first branch.

18. Apparatus for magnetic. sound recording comprising a control circuit for a magnet including a mixing circuit having a plurality of branches, means for applying to one branch of said mixingcircuit a sound modulated current which varies in magnitude and direction, means for applying to another branchof said mixing circuit a separate current the direction of which is instantaneously arithmetically additive to the current .in said first branch.

19. Apparatus for magnetic sound recording comprising. a control circuit for a magnetincluding a mixing circuit having a plurality of branches, means for applying'to one branch of said mixing circuit a sound modulated current which varies in magnitude and direction, means for applying .to another branch of said mixing circuit a separate current .of constant magnitude the direction of which is instantaneously arithmetically additive to the current in said first branch.

2 0. In'a recorder for magnetically recording sound which comprises a recording head provided with an electro-magnet having spaced poles adapted to be engaged by a continuously advanced record element formed of permanent magnet material, andwindings for said electromagnet;- a circuit for connection to the windings 9 comprising means for supplying a first voltage of variable magnitude and reversing polarity corresponding to a sound pattern to be recorded,

other means for supplying a second voltage of v substantially constant magnitude and reversing polarity substantially in phase with the first voltage, and means responsive to said voltages for establishing a current in the windings which varies in accordance with the additive values of said voltages, the magnitude of the voltage supplied by said other means being suflicient to cause a current in the windings substantially equal in magnitude to the coercive force of a record element to be magnetized.

LAWRENCE H. CONNELL.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 2,165,764 Pitsch July 11, 1939 2,265,831 Woolridge Dec. 9, 1941 2,403,540 Meneley July 9, 1946 2,440,553 Morrison Apr. 27, 1948 OTHER REFERENCES Radio Engineering, Terman, 3rd, page 188. 

